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roONLYycoLOWERCAneSEjo at gmail dot com ¶
18 years ago
Roy Conejo says:

I' had to "per-pixel alpha blend" an image into a solid background, as seen on the very concise example from "barnabas at kendall dot NOSPAM dot net": an alpha blended .png logo on a .jpg photograph. The problem was... it doesn't worked out at all here (why? T_T).

Note that I'm using just the source alpha to determine how much colour from source and destination will be present on the final pixel... and that I've to multiply the alpha value (0 - 127) by 2 because I need it to be 8 bits for the calculations.

I think the code is pretty fast, no decimals, no rounding, no unnecesary coding. Bound checking or clipping could be implemented if you really need to.

I hope it helps someone on my same situation ^.^

<?php

    ## BEGIN of function alpha_blending -------------------------------------------
    function
    alpha_blending ($dest, $source, $dest_x, $dest_y) {
    
        ## lets blend source pixels with source alpha into destination =)
        for ($y = 0; $y < imagesy($source); $y++) {
            for ($x = 0; $x < imagesx($source); $x++) {
                            
                $argb_s = imagecolorat ($source    ,$x            ,$y);
                $argb_d = imagecolorat ($dest    ,$x+$dest_x    ,$y+$dest_y);
                            
                $a_s    = ($argb_s >> 24) << 1; ## 7 to 8 bits.
                $r_s    =  $argb_s >> 16     & 0xFF;
                $g_s    =  $argb_s >>  8    & 0xFF;
                $b_s    =  $argb_s            & 0xFF;
                                
                $r_d    =  $argb_d >> 16    & 0xFF;
                $g_d    =  $argb_d >>  8    & 0xFF;
                $b_d    =  $argb_d            & 0xFF;
                                
                ## source pixel 100% opaque (alpha == 0)
                if ($a_s == 0) { 
                    $r_d = $r_s; $g_d = $g_s; $b_d = $b_s;
                } 
                ## source pixel 100% transparent (alpha == 255)
                else if ($a_s > 253) {
                ## using source alpha only, we have to mix (100-"some") percent 
                ## of source with "some" percent of destination.
                } else {
                    $r_d = (($r_s * (0xFF-$a_s)) >> 8) + (($r_d * $a_s) >> 8);
                    $g_d = (($g_s * (0xFF-$a_s)) >> 8) + (($g_d * $a_s) >> 8);
                    $b_d = (($b_s * (0xFF-$a_s)) >> 8) + (($b_d * $a_s) >> 8);
                }
                                
                $rgb_d = imagecolorallocatealpha ($dest, $r_d, $g_d, $b_d, 0);
                imagesetpixel ($dest, $x, $y, $rgb_d);
            }
        }
    }
    ## END of function alpha_blending -------------------------------------------
    
    
    
    ## let's start loading images as usual...
    $source = imagecreatefrompng ('logo.png');
    $dest    = imagecreatefromjpg ('photo.jpg');

    ## alpha blend $source into $dest starting at 10, 5.
    alpha_blending ($dest, $source, 10, 5);    
    
    ## here you'll have to save "$dest" or send it to the browser...
    ## 
    
    imagedestroy ($source);
    imagedestroy ($dest);
    
?>

eof =p
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